1. Field of the Invention
The present invention relates to a ferrite core for RFID (Radio Frequency Identification) application, a method of manufacturing the same and a ferrite coil that uses the same, and particularly to a ferrite core for RFID application that can be preferably applied to a device for detecting the air pressure or temperature of a vehicle tire, a vehicle theft preventing device, keyless entry system of a vehicle or the like, a method of manufacturing the same and a ferrite coil that uses the same.
2. Description of Related Art
Sintered ferrite has been used widely in various electronics apparatuses as the magnetic core of ferrite coil, transformer, magnetic head or the like. For example, such a sintered ferrite has been disclosed in Patent Document 1 containing at least one element selected from Mg, Ni, Cu, Mn and Li and has carbon content less than 96 ppm.
Patent Document 2 discloses a sintered ferrite containing 46 to 52 mol % of Fe2O3, 28 to 36 mol % of NiO and 16 to 22 mol % of ZnO as major components, wherein the proportion of crystal grains having sizes in a range from 0.2D to 3D is 50% by volume or more, D being the mean crystal grain size.
Further, Patent Documents 3 to 5 disclose a Ni—Zn-based sintered ferrite containing 48.0 to 50.0 mol % of Fe2O3, 14.0 to 24.0 mol % of NiO and 28.0 to 36.0 mol % of ZnO wherein mean crystal grain size is in a range from 3 to 30 μm and number of crystal grains having sizes exceeding twice the mean crystal grain size is within 10% of the total number of crystal grains.
Further, Patent Document 6 discloses a low-loss oxide magnetic material made of Ni—Zn—Cu ferrite containing Fe2O3, NiO, ZnO and CuO as main components wherein D50 in the crystal grain size distribution of the sintered body is from 8 to 31 μm, D10 is 3 μm or larger, and D90 is 50 μm or smaller.
Notification of the air pressure or temperature of a vehicle tire to the driver by means of RFID is carried out as follows. The tire is provided with a sensor that measures the air pressure or temperature of the tire by means of induced electromotive force of a ferrite coil generated by weak electromagnetic wave transmitted to the ferrite coil that comprises a magnetic material such as sintered ferrite and a coil wound thereon. When an electrical signal is transmitted to the sensor directing it to measure the pressure or temperature, the sensor measures the pressure or temperature in the tire and sends the measured data to the driver. Such technologies that employ the RFID are disclosed in Patent Documents 7 to 12.
The Patent Documents include such technologies related to a configuration of setting an RFID tag that uses ferrite magnetic material (Patent Document 7), a method for detecting air pressure in a tire using a magnetic material that is movable in a tire (Patent Document 8), an antenna for tire monitoring apparatus that uses a magnetic circuit based on ferrite magnet (Patent Document 9), a tire inner condition monitoring apparatus that generates an electromotive force in a coil comprising a magnetic material as the core and detects abnormal rotation of the tire (Patent Document 10), a configuration of mounting a tire pressure detecting apparatus that uses a ferrite coil (Patent Document 11) and a low tire pressure warning apparatus that uses a magnet coil (Patent Document 12).
Further, Patent Document 13 proposes a loop antenna apparatus that allows for keyless entry to a vehicle (non-contact locking and unlocking of vehicle doors) by making use of induced electromotive force generated in a ferrite core for RFID application comprising a Ni—Zn-based ferrite core with an electrically conductive wire wound thereon.
However, it is difficult to measure the air pressure or temperature in the tire by using the sintered ferrite disclosed in Patent Documents 1 to 6 as the ferrite or magnetic material used in the RFID technology disclosed in Patent Documents 7 to 13. This is because a sufficient level of induced electromotive force cannot be generated in the ferrite coil that uses the sintered ferrite disclosed in Patent Documents 1 to 6, and therefore a sufficient signal directing the pressure sensor or the temperature sensor to measure the pressure or temperature in the tire is not transmitted to a pressure sensor or a temperature sensor. When the sintered ferrite disclosed in Patent Documents 1 to 6 is used to make the ferrite coil of the loop antenna apparatus of Patent Document 13, it is difficult to lock and unlock the vehicle door. Difficulty in measuring the air pressure or temperature in the tire or in locking and unlocking the door is caused because sufficient induced electromotive force is not generated in the ferrite coil made of the sintered ferrite of Patent Documents 1 to 6. Thus there has been a demand for ferrite core for RFID application that can generate a sufficient level of induced electromotive force. The ferrite core for RFID application is also required to have high mechanical strength that does not vary significantly, in order to improve the safety of vehicles. But the conventional sintered ferrite materials do not satisfy these requirements.
A ferrite coil made of the sintered ferrite of Patent Documents 1 to 6 may have low sensitivity per one winding that is determined by dividing the induced electromotive force, generated in the ferrite coil when a magnetic field is applied thereto, by the intensity of the applied magnetic field, thus giving rise to such a problem that the sintered ferrite core used for the ferrite coil must be made larger and the number of windings must be increased, resulting in difficulty to make the ferrite coil smaller. There has also been such a problem that making the ferrite coil smaller leads to lower level of induced electromotive force.
Causes of the decrease in the mechanical strength and the decrease in the induced electromotive force are as follows. In order to obtain a ferrite coil having high and stable levels of induced electromotive force and sensitivity, it is necessary to use a ferrite core that has low core loss, high magnetic permeability that does not vary significantly and high Curie temperature. To make a ferrite core having low core loss, it is necessary to order the crystalline structure of the crystal phase of spinel type (one or more kinds of solid solutions selected from NiFe2O4, ZnFe2O4 and FeFe2O4) contained in the ferrite core. By ordering the crystalline structure, it is made possible to make a ferrite core that has lower core loss, higher and more stable magnetic permeability and higher Curie temperature from the material of the same chemical composition (for example, contents of Fe, Ni and Zn). Crystalline structure of the ferrite core made of the conventional sintered ferrite is sometimes poorly ordered, thus resulting in large core loss, low magnetic permeability and low Curie temperature. In such a case, the ferrite core for RFID application that uses the ferrite core made of the conventional sintered ferrite has insufficient level of induced electromotive force.
Moreover, making a sintered ferrite having high mechanical strength requires it to order the crystalline structure of spinel type crystal phase so as to decrease the internal stress and control the variations in the crystal grain size within a certain range. It is made possible to make a sintered ferrite that has higher and more stable magnetic permeability and mechanical strength from the material of the same composition (for example, contents of Fe, Ni and Zn) by such control.
However, with the conventional sintered ferrite that has such trouble as the crystalline structure is poorly ordered and/or the crystal grain size is not controlled, it has not been possible to achieve sufficiently high mechanical strength on stable basis.
Moreover, since tires may be exposed to intense mechanical vibrations at high temperatures, the sintered ferrite that constitutes the ferrite core for RFID application is required to have not only low core loss, high Curie temperature and high magnetic permeability, but also small relative temperature coefficient of magnetic permeability in absolute value and highly dense structure, depending on the application. Moreover, in case the ferrite core provided with an electrically conductive wide wound thereon is molded with a resin, it has been required to decrease the changes in inductance in the presence of compressive stress, since the molding process generates the compressive stress in the ferrite core. The sintered ferrite that constitutes the ferrite core disclosed in the prior art cannot satisfy these requirements.
The sintered ferrite of Patent Document 1 has crystal grain size distribution that is not controlled, although it is claimed that the mechanical strength can be improved by controlling the carbon content to less than 96 ppm. While it is described that mean crystal grain size is preferably controlled within a range from 1 to 30 μm, there is a possibility that magnetic permeability and/or Curie temperature become lower when the crystal grain size distribution varies. Further, since there is no description about carefully crushing the calcined material, unsatisfactory characteristics sometimes result such as large core loss, low magnetic permeability, low Curie temperature and/or low mechanical strength, in case the particle sizes of the calcined powder are larger which leads to insufficiently ordered crystalline structure of the sintered body.
Patent Document 2 specifies the crystal grain size distribution of the ferrite material, but a large core loss sometimes results because the particle sizes of the powder are larger before calcination which leads to larger size of crushed powder after calcination and to crystalline structure that is not ordered.
The sintered ferrite of Patent Documents 3 to 5 has crystal grain sizes uniformly controlled, although the particle sizes of the powder before calcination and the particle sizes of crushed powder after calcination are not controlled. As a result, the crystalline structure of the sintered body is not sufficiently ordered, thus resulting in large core loss, low magnetic permeability, low Curie temperature and/or low mechanical strength.
Patent Document 6 discloses a low-loss oxide magnetic material made of sintered ferrite that has power loss (core loss) of the sintered ferrite decreased by defining the calcination (preliminary firing) temperature and the particle size of the calcined powder. However, larger core loss sometimes results due to the following causes. In case particle sizes of the powder before calcination are large in the process of manufacturing the sintered ferrite, it may become impossible to sufficiently synthesize the calcined powder, namely a powder that has spinel structure and consists of crystal grains having sufficiently ordered crystalline structure. Cause of insufficient synthesis is that powders of Fe2O3, NiO, ZnO and CuO do not react sufficiently in the calcination process, thus resulting in insufficient growth of the crystal grains of spinel structure, and therefore much of the powders of Fe2O3, NiO, ZnO and CuO remain in the state of calcined powder without reacting. When the calcined powder containing much non-reacted powder is sintered, crystalline structure of the crystal phase having spinel structure cannot be ordered enough during the sintering process. Therefore, when particle sizes of the powder before calcination are large, much crystal of which structure is not sufficiently ordered remains in the sintered body, thus resulting in large core loss. Even when the calcined powder is synthesized sufficiently, insufficient crushing of the calcined powder results in large particles contained in the powder before calcination, and therefore crystalline structure of the sintered body is not sufficiently ordered and core loss becomes larger. When calcined, the powders of Fe2O3, NiO, ZnO and CuO react with each other so as to generate crystal grains having spinel structure. However, it is supposed that the crystal grains that constitute the calcined powder do not have exactly the same composition. Slight differences in the lattice constant between the crystal grains in the sintered body suggest that the crystal grains that constitute the calcined powder have different compositions according to variations in the compositions. The greater the variations in the composition, the more the lattice defects generated in the crystal of spinel structure, resulting in irregular crystalline structure. Suppressing the occurrence of such lattice defects contributes to the ordering of crystalline structure in the crystal phase of spinel structure. This is because insufficient crushing of calcined powder causes the composition of the crystal grains contained in the sintered body to vary, and sintering of the material having the varying compositions not corrected leads to variations in the composition of the crystal grains contained in the sintered body, thus resulting in the sintered body having crystalline structure that is not ordered enough. Thus the low-loss oxide magnetic material of Patent Document 6 sometimes has large core loss.
As described above, since the sintered ferrite or the ferrite material of Patent Documents 1 to 6 does not have low core loss, high magnetic permeability and/or high Curie temperature, the ferrite coil made from such sintered ferrite or the ferrite material has a problem of low induced electromotive force generated by each winding when subjected to a magnetic field.
Thus it is difficult to apply the ferrite coil made from the sintered ferrite or the ferrite material of Patent Documents 1 to 6 to ferrite core for RFID application that is required to have large induced electromotive force, high sensitivity and good mechanical properties.
In any of the magnetic material used in the configuration of setting RFID described in Patent Document 7, the magnetic material used in the detection method disclosed in Patent Document 8, the ferrite magnet used in the antenna described in Patent Document 9, the magnetic material used in the monitoring apparatus of Patent Document 10, the ferrite magnet used in the configuration of Patent Document 11, the magnetic material used in the pressure warning apparatus of Patent Document 12 and the Ni—Zn-based ferrite magnet used for the antenna of Patent Document 13, there is no description about controlling the composition of the magnetic core, crystal grain size distribution and the crystalline structure, and therefore there has been such problems as large core loss, high magnetic permeability and/or low Curie temperature. Thus it has been difficult to apply these materials to ferrite core for RFID application.    Patent Document 1: Japanese Unexamined Patent Publication (Kokai) No. 2001-93718    Patent Document 2: Japanese Unexamined Patent Publication (Kokai) No. 2002-179460    Patent Document 3: Japanese Unexamined Patent Publication (Kokai) No. 2001-15322    Patent Document 4: Japanese Unexamined Patent Publication (Kokai) No. 8-310856    Patent Document 5: Japanese Unexamined Patent Publication (Kokai) No. 8-310855    Patent Document 6: Japanese Unexamined Patent Publication (Kokai) No. 2002-343621    Patent Document 7: Japanese Unexamined Patent Publication (Kokai) No. 2002-264617    Patent Document 8: Japanese Unexamined Patent Publication (Kokai) No. 3-28007    Patent Document 9: Japanese Unexamined Patent Publication (Kokai) No. 2-263137    Patent Document 10: Japanese Unexamined Patent Publication (Kokai) No. 3-292207    Patent Document 11: Japanese Unexamined Patent Publication (Kokai) No. 9-309305    Patent Document 12: Japanese Unexamined Patent Publication (Kokai) No. 2-162222    Patent Document 13: Japanese Unexamined Patent Publication (Kokai) No. 2001-308629